Lithium ion secondary batteries have excellent properties such as high energy density, high operating voltage, and less deterioration caused by charge and discharge cycle(s), and therefore have been widely used for portable information terminals including cellular phone and notebook computer, and for industrial equipment. Their applications to electric vehicle, electric power storage, and the like, which may be realized by a larger lithium ion secondary battery, is expected in the future, and researches and developments are conducted for the applications.
At present, graphite is commonly used as a negative electrode active material for lithium ion secondary batteries. However, when graphite is used as a negative electrode active material, the decomposition reaction of the electrolyte may occur at the electrode/electrolyte interface, resulting in the reduction in capacity.
In order to solve the problem, Patent Literatures 1 and 2, for example, propose that graphite be coated with amorphous carbon or low-crystalline carbon. That may prevent direct contact of the electrolyte with the surface of graphite, thereby suppressing the decomposition reaction of the electrolyte.